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You don't repair software. It's blatantly written on the warranty card that came in the box with the installer. You can be held criminally liable if you insist on doing so.

Removing viruses is just a service and does not constitute software repair. Reinstalling would need the software's license key which the owner should supply to you. Uninstalling software using a license key which was not bought constitutes software piracy.

I doubt you can repair opensource freeware Linux programs. I don't see the need either. I haven't seen a Linux virus before despite using Linux for the past 10 years.

You can offer those services to your classmates. I suggest you look at the service pricing at computer repair shops and lower them a bit to attract customers. Just thinking ahead though. What will you do if you inadvertently deleted everything in the hard disk and there was something very valuable in there? Would you have enough funds to bribe the customer from not suing you?